1. Field of the Invention
The present invention relates to a chemical reactor for heterogeneously catalysed reaction of a fluid with an improved holding device for inert particles and/or catalyst particles through which the fluid flows. It furthermore relates to a process for carrying out chemical reactions by means of such a reactor and to the use of knitted wire mesh fabric in chemical reactors.
2. Description of Related Art
The hydrogenation of nitroaromatics to give the corresponding amines in a fixed bed reactor with axial flow can be carried out in a multi-stage process under adiabatic reaction conditions. Such hydrogenations proceed in periodic cycles, production cycles alternating with regeneration cycles in which, for example, carbon-containing deposits are removed by burning off.
Reactors for such reactions typically have a cylindrical shape and are equipped with a slotted screen which serves as a base for the fixed bed catalyst (Heterogeneous Catalysis in Industrial Practice, 2nd ed. 1996, page 474). These slotted screens must carry the weight of the catalyst and the additional load resulting from the pressure loss over the bulk catalyst.
Slotted screens such as are described, for example, in U.S. Pat. No. 2,915,375 are often used in this context. They comprise V-shaped wires arranged in parallel, which in their turn are supported again and fixed so that slots result, the width of which is smaller than the particle size of the material to be supported and which widen downwards, in order to lead away downwards any material which gets between the wires and to avoid blocking of the slotted screen.
Due to the periodic operation of the reactors, the screen is subjected to thermal stresses which lead to damage. This is the case in particular if exothermic reactions are carried out under adiabatic conditions, that is to say the heat of reaction liberated is taken up by the reaction gas, so that large jumps in temperature occur insider the reactor.
In these cases deformations in the V-shaped wires originally arranged in parallel can already occur after a few production cycles, up to breaking off or tearing of individual wires. The repair of a screen damaged in this way requires outlay and does not bring about permanent success. To minimize these problems, the slotted screens are as a rule fixed to a supporting ring mounted in a floating manner, which leaves a gap to the reactor wall in order to compensate thermal expansions.
Precisely in the case of larger reactors is an absolutely flat contact surface (support ring) to be manufactured for this ring, if at all, only with considerable outlay. Furthermore, it is difficult to manufacture exactly circular large reactors. As a consequence of these problems, a loss of catalyst often occurs in the form of the catalyst trickling through damaged areas of the slotted screen or through openings between the slotted screen and reactor wall or support ring.
The problems are intensified further by the use of ever smaller catalyst particles, which in general display a higher efficiency but also trickle more easily through small defects. On the surface of the bulk thrombus-shaped depressions arise due to loss of catalyst. The reaction gas then flows preferentially through the regions with the depressions, so that a non-uniform gas flow develops, up to bypassing of unreacted educt. The catalyst which has migrated to the reverse of the slotted screen can furthermore cause damage to the apparatuses downstream of it.
In order to avoid passage of catalyst through to the other side of the slotted screen, the catalyst can be mounted on a complex, multi-layered bulk of inert particles, the particle diameter in the layer closest to the catalyst being somewhat greater than the particle diameter of the catalyst and then increasing with each layer (Fixed-Bed Reactor Design and Diagnostics, 1990, FIG. 1.1, page 4).
By this means, quite large inert particles which cannot yet trickle through the screen at relatively small defects initially lie on the slotted screen. Disadvantages of this solution are on the one hand that a not inconsiderable part of the reactor is filled with inert material and is no longer available for the actual catalyst, and on the other hand that the introduction of the various layers into the reactor requires outlay. If the inert material is to be re-used when the reactor is filled again, this requires a multi-stage sieving of the material removed from the reactor in order to separate the particles of different sizes from one another.